This invention relates to a novel self-curing resin which contains, in the molecule, reactive groups complementary to each other, namely a blocked isocyanate group and a hydroxyl group and which is useful as an adhesive or paint, for instance.
Two-component (two-can) or one-component (one-can) type paint systems comprising a hydroxyl-containing polymer, such as an acrylic polyol, and an isocyanate or blocked isocyanate, which is a crosslinking agent or curing agent, can form paint films excellent in chemical resistance, physical properties, weather resistance and so forth and have hithertofore been used widely as automobile paints and so on.
Among them, however, two-component systems have drawbacks. For example, they raise health and safety problems on the occasions of isocyanate crosslinking agent incorporation and paint application, in particular. The paint prepared by admixing the two components has a short pot life (working life); the paint may acquire an excessively increased viscosity during paint application and/or paint applicators may become difficult to wash, for instance. One component systems also have their drawbacks. Since the compatibility between blocked polyisocyanate compound (crosslinking agent) and hydroxyl-containing polymer is low, they give paint films poor in curability, luster and surface smoothness.
Unexamined Japanese Patent Publication No. 186722/1988, among others, discloses, as a measure for removing these drawbacks, a self-curing resin produced by copolymerizing an unblocked or blocked isocyanate-containing, radical-polymerizable monomer and a hydroxyl-containing, radical-polymerizable monomer and, hence, containing complementary reactive groups in the molecule.
However, by this measure, it is difficult to prevent the reaction between the isocyanate group and the hydroxy group even when the copolymerization reaction between the unblocked isocyanate-containing, radical-polymerizable monomer and the hydroxyl-containing, radical-polymerizable monomer is conducted at a temperature as low as possible. Gelation tends to occur readily during the polymerization reaction. Thus, it is difficult to conduct the polymerization to a sufficient extent while inhibiting gelation. On the other hand, when a blocked isocyanate-containing monomer is used, the tendency toward gelation during polymerization indeed decreases as compared with the above. However, phenol- or oxime-blocked isocyanate-containing monomers, for instance, require a high temperature of 120.degree.-140.degree. C. or above for polymerization and, therefore, the blocking agents may undergo dissociation during polymerization, disadvantageously leading to gelation and at the same time to marked polymer discoloration. Furthermore, polymers derived from blocked isocyanate-containing monomers in which other blocking agents are used require a crosslinking/curing temperature of 150.degree.-170.degree. C. or above, which is higher than that for ordinary heat-curable resins, since the dissociation temperature for said blocking agents is high. Hence, a self-curing resin, which is curable at low temperatures (not higher than 120.degree. C.), can scarcely be obtained.
It is an object of the invention to provide a novel self-curing resin free of the drawbacks mentioned above.
Another object of the invention is to provide a novel self-curing resin which can be readily produced by polymerization without gelation or discoloration and can be cured at low temperatures.
These and other objects of the invention will become apparent from the description which follows.